Re: [PATCH 5/5] fs: Convert struct file::f_count to refcount_long_t

[Christian Brauner]

On Thu, May 02, 2024 at 05:41:23PM -0700, Kees Cook wrote:
> On Fri, May 03, 2024 at 01:14:45AM +0100, Al Viro wrote:
> > On Thu, May 02, 2024 at 05:10:18PM -0700, Kees Cook wrote:
> > 
> > > But anyway, there needs to be a general "oops I hit 0"-aware form of
> > > get_file(), and it seems like it should just be get_file() itself...
> > 
> > ... which brings back the question of what's the sane damage mitigation
> > for that.  Adding arseloads of never-exercised failure exits is generally
> > a bad idea - it's asking for bitrot and making the thing harder to review
> > in future.
> 
> Linus seems to prefer best-effort error recovery to sprinkling BUG()s
> around.  But if that's really the solution, then how about get_file()
> switching to to use inc_not_zero and BUG on 0?

Making get_file() return an error is not an option. For all current
callers that's pointless churn for a condition that's not supposed to
happen at all.

Additionally, iirc *_inc_not_zero() variants are implemented with
try_cmpxchg() which scales poorly under contention for a condition
that's not supposed to happen.